Active range finders based on triangulation have proven to be very useful when dense depth maps are required. Applications such as robotics systems, adaptive welding, marine propeller blade measurement, and the measurement and reproduction of objects in three dimensions have benefitted from the technique. See "Practical Considerations for a Design of a High Precision 3-D Laser Scanner System" by F. Blais et al published in Optomechanical and Electro-Optical Design of Industrial Systems, SPIE Proceedings, 1988.
Most high precision range finders use a solid state line scanner arranged as a linear array of photodetectors. To measure the position of the reflected spot, the pixels in the array are scanned sequentially at a clock speed ranging from 2 to 20 MHz. High depth resolutions can be achieved by feeding the output of the array into a numerical peak detection circuit.
Area sensors, which include imaging tubes and solid state arrays are the most common imaging devices and can be purchased in large volume. The most common types of solid state imaging sensors used in robotics are the video format CCD or CID bidimensional imaging devices having something of the order of 450.times.480 pixels. Such sensors typically have a dynamic range of 10.sup.3.
Linear solid state sensors (CCD, CID) and photodiode arrays are similar in characteristics to the previous devices. They are used with single point range sensors and provide high performance position detection. These sensors typically have a dynamic range of 5.times.10.sup.3.
Point detectors such as lateral effect photodiodes are used mainly for very fast single point 3-D range sensors. The dynamic range of a lateral effect photodiode is typically 10.sup.4.
All these position sensitive detectors require external electronic circuits to evaluate the position of the spot produced by the incident laser beam. The resulting dynamic range of the whole system is usually limited to about 10.sup.2 because of the limitations of the analog-to-digital converters.
Among the photodetectors available commercially, the most important ones are the photodiode, the avalanche photodiode, and the photomultiplier tube. The last two are used mainly to detect very weak signals. For laser intensity control, a photodiode is sufficient for most applications, because the control system should respond to light intensities above the saturation level of the position sensor. Weak signals are taken care of by the dynamic range of the position sensor. The dynamic range is typically 10.sup.4.
A practical limitation of all these sensors is their relatively low saturation level. Once the saturation exposure (saturation charge) has been reached over many pixels, the accuracy drops to an unacceptable level. For example, if the object being inspected is a polished metal surface (as often found on printed circuit boards), such a surface can reflect a projected laser beam almost completely to the sensor, if the surface is oriented within the viewing angle of the sensor. On the other hand, if the orientation of the surface falls outside the viewing angle, only the small scattering lobe of light will be received. This results in a very wide dynamic range of received intensities, typically 10.sup.5 to 10.sup.6, whereas, as explained above practical photodetector arrays usually only have a dynamic range of 10.sup.2 to 10.sup.3, or at the most 10.sup.4.